Calibrating an antenna consists in determining for all the useful directions the estimation error (bias and standard deviation) between the real direction of arrival of a signal received by the antenna and the direction of arrival estimated at the output of the antenna processing; the values measured are thereafter corrected upon utilization of the antenna by compensating for the corresponding bias. The knowledge of the bias in particular is a data item indispensable to the holding of the performance of the equipment associated with the antenna. In regard to the measurement of the bearing of the source, the equipment acts as a measurement apparatus whose antenna constitutes the sensor. A bias in the determination of the bearing can impair the tactical operation of the system and in particular the locating function. This is why it is generally useful to undertake a calibration.
To calibrate an antenna use is made of a noisemaker, preferably fixed, but usually mobile for operational reasons, in front of which the receiver is made to deploy in such a way that all the bearings of the span to be calibrated are explored: the source-receiver geometry must permanently be known perfectly (or moreover measured with an accuracy noticeably greater than the required accuracy for the receiver to be calibrated) in order for it to be possible to make the comparison between real bearing and estimated bearing on the reception antenna. The determination of the real bearing presupposes either its very accurate measurement by a procedure available during calibration or that it is possible to associate (temporally) signal emitted and signal received and therefore that it is possible to define the axis joining the position of the emitter to this of the antenna, corresponding to the measurement. It is necessary to specify in this regard that in principle propagation intervenes and that if one considers a signal emitted at the instant t it will be received at an instant t+ΔT (such that ΔT=D/c, c being the speed of sound in water and D being the distance traversed by the signal between source and reception antenna). For the calibration, the angle of arrival, measured on reception at t+ΔT, must theoretically be compared with the direction of the source axis at the date t and antenna at the date t+ΔT. This precaution is often neglected, the error that is made then being small since it is equal to v/1500 radians, v being the speed of motion of the source in m/s.
In the case of the calibration of sonar antennas carried by submarines, these two conditions impose constraints of implementation of the measurements which are difficult to satisfy, whatever method is used.
A first method commonly used in the past, consists in performing the calibration with the aid of a periscope. The calibration then consists in performing optical measurements of angles by means of the periscope of the submarine carrying the antenna to be calibrated and in comparing the angle measurement obtained by means of the periscope with the measurement provided by the sonar receiver associated with the antenna.
This first procedure has the advantage of simplicity. Specifically, it does not require any complementary measurement equipment, the periscope forming part of the onboard equipment. On the other hand, this procedure has several drawbacks.
Measurement by periscope firstly requires a prior operation of calibration of the periscope which must serve as measurement reference.
Measurement by periscope thereafter requires that the submarine remain just below the surface with the consequence, sometimes, that the antenna to be calibrated is incompletely immersed, thereby modifying its characteristics. Moreover, even if the antenna is completely immersed, the conditions of propagation of the sound waves under the surface are very particular and subject to meteorological vagaries, and this may impair the measurements. This method does not allow calibration of the antenna at various depths, this being necessary in order to take into account the variations in acoustic behaviour of the antenna as a function of pressure.
Periscopic sighting is, in good visibility conditions, a fairly accurate measurement. Nevertheless, to make an accurate measurement, it is necessary to be able to sight the source itself, this not being possible if the latter is hauled behind a boat or carried by a submarine vehicle.
On account of the constraints cited above, the method of calibration by periscope allows only imperfect calibration in overly constraining operational conditions. This is why it is often reserved for preliminary checks.
The arrival of modern electromagnetic positioning means using in particular satellite-based locating means, of GPS type for example, have upgraded the calibration methods based on the knowledge of the absolute positions of the noisemaker and of the receiver by removing in particular the depth and proximity constraints. With such locating means, the towing ship can ascertain its initial position with sufficient accuracy and refresh its position information throughout the duration of the calibration operation. Likewise, the submarine can ascertain before diving, its initial position, position information that it will be able to update over time by means of its onboard navigation instruments. It can thus use its inertial system (SINS) or else navigate by dead reckoning by means of the gyrocompass and the log. Thus, during the calibration operation the receiver performs angular measurements βm(ti) and associates them with the coordinates of the submarine, calculated for the measurement instants ti.
To carry out the calibration operation with such means, it remains to associate for each measurement instant ti the information of position of the noisemaker, available aboard the towing ship, with the information of position of the submarine and with the corresponding measurements βm(ti), which are recorded aboard the submarine. This association is generally carried out on land by sifting of the recordings performed during the measurement operation.
With respect to the methods using a sighting by means of a periscope, previous case, this second type of method presents the advantage, as was stated previously, of enabling the noisemaker to be made to deploy at a sufficient distance to minimize the influence of parallax. Moreover, the submarine no longer being constrained by the need to raise its periscope, it is possible by virtue of this type of method to perform calibration measurements for various depths under pressure conditions that are more operational.
On the other hand the measurements being carried out independently on each vessel, this type of method does not enable the result of the calibration to be ascertained immediately on completion of the measurement operation. The sifting of the data, generally carried out on land, can take several weeks. This interlude can bring about a delay in the validation of the operational functionalities utilizing the azimuth measurements arising from the sonar.
This drawback can be partially circumvented by furnishing the towing ship and the submarine with means of acoustic communication allowing for example the towing ship to transmit its position in real time to the submarine during the measurement operation. However, these specific acoustic means are unwieldy to implement and expensive. Moreover, it may produce appreciable disturbances of the angle measurement.
It is also possible to envisage after the measurements phase, a phase of data transmissions between the towing ship and the submarine, the submarine resurfacing to establish a radio link with the towing ship, during which link the towing ship transmits its position file. The whole set of operations can then be processed thereafter aboard the submarine. In practice, this second solution presupposes that the towing ship, which is in general only a support vessel of the port institutions, is furnished with digital transmission means compatible with those of the submarine and having a sufficient throughput.
In all cases, this type of method, which involves GPS type means associated with various secure communication means, mobilizes a fairly unwieldy infrastructure. Moreover, they are inoperative in the event of a noisemaker carried by a submarine.
As regards known calibration methods it is finally possible to cite the methods using a fixed source of known position, for example anchored to the bottom. In practice, the use of such a method is very limited and can be applied only to configurations of shallow water type. In deep waters, the anchoring of a fixed source is difficult and to keep a towed noisemaker fixed is hardly conceivable. Towing ships preferring, for maritime safety reasons, not to remain at high tide with their engines stopped, move thereby causing movement of the noisemaker. The use of boats with dynamic positioning such as those of oil operations, moreover very noisy and not very widespread, does not offer any real solution to the problem of keeping the source in a fixed position at high tide.
It is noted therefore that the known prior art calibration methods, based on the knowledge by the receiver of the absolute positions of the noisemaker and of the reception antenna throughout the duration of the measurements, all exhibit significant drawbacks.